Kdt26417 gave you a lot of good resources, but I'll also say that a good therapist/counselor can help you work within your relationship, regardless of the structure. They don't have to see it by mono or Poly 'rules' if you don't let them. Think about what your goal is with counseling, focus on that, and a good counselor will be able to help you.
I go to regular therapy because of my bipolar disorder and Lamb and Pickle are a major part of that. We work on our relationship within the parameters of my illness and how it affects them. Both my mental state and our relationship have benefitted greatly... And this from a therapist whose area of expertise is mental health, not relationship, not poly relationship.
I think if you can't find a poly specific therapist, you just shop for someone who makes you feel real progress over reasonable periods of time.